1. Field of the Invention
The present invention relates to vibration-utilizing linear actuators that can be applied to driving of fingertips of a robot hand or the like.
2. Description of the Related Art
Regarding actuators using a vibrator and a piezoelectric element, some configurations, such as an ultrasonic motor, are known. In particular, realization of movement of fingertips of a robot hand using the actuators requires realization of a small linear motion mechanism, which corresponds to the human muscle.
Japanese Patent Publication No. 4-72471 discloses a structure of a rotary ultrasonic motor. Realization of movement of fingertips of a robot hand with a rotary motor disclosed in Japanese Patent Publication No. 4-72471 requires a rotary-to-linear-motion converting mechanism, such as a screw, which makes it difficult to reduce the size.
In that respect, since a linear actuator using ultrasonic vibration can realize linear movement with a small structure, the actuator is preferable for controlling fingertips of a robot hand. In particular, a long and thin tubular (cylindrical) linear actuator utilizing vibration of a piezoelectric element is capable of handling an increase in the speed of movement of a robot, an increase in the stroke, and an increase in the accuracy. Progress has recently been made in a study of such a linear actuator.
The following describes such a cylindrical linear actuator.
FIGS. 14A and 14B show an ultrasonic motor of rotary-and-linear-motion-integrated type and an electronic device including the same as disclosed in Japanese Patent Laid-Open No. 10-210776. The ultrasonic motor includes a cylindrical stator 101 and a cylindrical mover 102 in contact with an inner or outer surface of the stator 101. The stator 101 has a cylindrical piezoelectric element 103, a plurality of polarization electrodes 104 regularly arranged on one of the inner surface and the outer surface of the piezoelectric element 103, and a whole surface electrode 107 provided on the other surface. The mover 102 is driven by ultrasonic vibration generated in the stator 101. In this case, the mover 102 can be driven freely in rotary and linear directions by selectively applying a plurality of kinds of out-of-phase alternating voltage to the polarization electrodes 104.
FIG. 15 shows an ultrasonic linear motor disclosed in Japanese Patent Laid-Open No. 5-49273. This ultrasonic linear motor includes a first piezoelectric element 203a that vibrates in parallel to the traveling direction, a shaft 201 that penetrates through the first piezoelectric element 203a, and second and third piezoelectric elements 203b and 203c that are arranged to sandwich the first piezoelectric element 203a and to be able to hold the shaft 201, and that vibrate vertically to the traveling direction. A gap is provided between the first piezoelectric element 203a and the shaft 201 in a radial direction. The second and third piezoelectric elements 203b and 203c are set so that a tightening margin and a gap exist between the piezoelectric elements 203b and 203c and an outside diameter of the shaft 201 at the time of shrinkage and expansion, respectively. The driving speed can be changed by changing a phase difference of alternating voltages applied to the piezoelectric elements.
In general, a vibration actuator according to the related art vibrates either the mover or the stator as a vibrator to generate a friction-based driving force (thrust) in the traveling direction at a contact portion of the mover and the stator.
In an ultrasonic motor of rotary-and-linear-motion-integrated type disclosed in Japanese Patent Laid-Open No. 10-210776, a mover can be driven freely in rotary and linear directions by selectively applying a plurality of kinds of alternating voltage to a piezoelectric element to vibrate the piezoelectric element.
Additionally, in an ultrasonic linear motor disclosed in Japanese Patent Laid-Open No. 5-49273, driving is realized in the linear direction by alternating voltage applied to a first piezoelectric element.
However, since the vibrator is constituted by the piezoelectric element in the ultrasonic motors disclosed in Japanese Patent Laid-Open Nos. 10-210776 and 5-49273, the following unsolved problems exist.
1) Design Flexibility
At the time of design and manufacture of an actuator utilizing vibration of a vibrator, the shape of the vibrator and the shape and frequency of a natural vibration mode are essential design parameters directly related to the thrust and the speed of movement. However, since a piezoelectric element is a sintered body, the piezoelectric element does not have a mechanical strength of metal, and mechanical processing methods therefor are also limited. Accordingly, in ultrasonic motors disclosed in Japanese Patent Laid-Open Nos. 10-210776 and 5-49273 which form a vibrator with a piezoelectric element, the design flexibility of realizing the large thrust and the high-speed movement decreases.
2) Durability of Vibrator
To realize the high-speed driving, a vibrator has to be vibrated at a significantly high speed in an actuator utilizing vibration. Accordingly, in view of the durability of the actuator, a material of the vibrator has to be resistant to repeated deformation and a structure of the vibrator is preferably simple and irrefrangible. In addition, the material of the vibrator has to have small internal damping in consideration of heat generated in the material.
Nevertheless, in ultrasonic motors disclosed in Japanese Patent Laid-Open Nos. 10-210776 and 5-49273 which form a vibrator with a piezoelectric element, since the vibrator has a complex structure in which electrodes sandwich the vibrator, and a large-amplitude vibration at a high frequency causes pealing of the electrodes, the vibrator thus cannot realize high durability. In addition, since the piezoelectric element has large internal damping, high-speed large-amplitude vibration undesirably increases an amount of generated heat.
3) Contact Force of Vibrator
In addition, since a friction force serving as a driving force is generated in an actuator utilizing vibration, a contact force for keeping a vibrator close to a stator is needed. If this contact force is too weak, the friction force, namely, the driving force, decreases. If the contact force is too strong, the force disturbs vibration of the vibrator and decreases the durability undesirably. Accordingly, it is important to keep the contact force constant in vibration actuators. In the case of cylindrical linear actuators, this contact force is compensated by a fitting accuracy of the vibrator and a circular tube serving as a stator.
The fitting accuracy of the vibrator and the circular tube significantly changes depending on heat generated in a piezoelectric element and a change in an atmosphere temperature. Thus, a piezoelectric material constituting the vibrator and a material of the stator or the mover preferably have thermal expansion coefficients that are as equal to one another as possible. However, since the circular tube is generally made of metal or the like, a significant difference exists between the thermal expansion coefficients when the vibrator is made of a piezoelectric material, which thus increases an influence of heat.
4) Vibration Amplitude and Movement Speed of Vibrator
In general, a deformation ratio of a piezoelectric element is substantially equal to 10−5. Accordingly, when a piezoelectric element having a diameter of 2 mm is used, deformation of only 2×10−5 mm=20 nm is caused. Since surface roughness of the piezoelectric element is much larger than 20 nm, realization of a small linear motion actuator having a diameter of 2 mm or the like is difficult.
In addition, the speed of movement in the traveling direction is equal to the product of the driving frequency and the amplitude. Therefore, a small vibration amplitude value equates to a slow movement speed. When the diameter of the piezoelectric element is 2 mm, the amplitude in the circumferential direction is 20 nm, and the component in the traveling direction is 1/10 thereof, the movement speed of the vibrator driven at 50 kHz is 20 nm× 1/10×50 kHz=0.1 mm/s, which is significantly slow.
The vibrator needs to be vibrated at a large amplitude to realize the high-speed movement. However, if the amplitude becomes too large, fluid, such as air existing in a gap adjacent to a contract portion, is compressed and pressure thereof undesirably levitates the vibrator (ultrasonic levitation). As a result, friction is not generated at the contact portion and the thrust cannot be obtained. That is, large-amplitude vibration undesirably decreases the thrust.